When my daughter was ten and a half months old, she qualified as “wasted,” which UNICEF describes as “the most immediate, visible and life-threatening form of malnutrition.” My wife and I had been trying hard to keep her weight up, and the classification felt like a pronouncement of failure. Her birth weight had been on the lower end of the scale but nothing alarming: six pounds, two ounces. She appeared as a dot on a chart in which colored curves traced optimal growth; fifteenth percentile, we were told. She took well to breast-feeding and, within a month, had jumped to the twentieth percentile, then to the twenty-sixth. We proudly anticipated that her numbers would steadily climb. Then she fell behind again. At four months, she was in the twelfth percentile. At nine and a half, she was below the fifth.
Our pediatrician was worried. Ease off the lentils and vegetable smoothies, we were warned; we needed to get more calories into our babe. Ghee, peanut butter—we were to drench her food in these and other fats and wash them down with breast milk and formula. And that’s what we did. When we came back a month later, though, we learned that she had dropped further—and crossed into “wasted” territory.
Was this what malnutrition looked like? She seemed to be flourishing. She was happy, adventurous, and exuberantly social, babbling incessantly and forever engaging strangers with flirtatious stares. She had cheeks as plump as the juicy clementines that she loved to eat with full-fat yogurt. Although slow to hands-and-knees crawling—scooting was her preferred means of locomotion—she was hitting most of her other milestones. She was also growing longer and longer, shooting from the twelfth percentile at birth to the thirty-sixth at ten months.
In “Adaptable: How Your Unique Body Really Works and Why Our Biology Unites Us” (Avery), Herman Pontzer, an evolutionary anthropologist at Duke University, recounts facing a similar conundrum. While Pontzer was visiting a semidesert village in northern Kenya to study the Daasanach pastoralists, a German charity representative told him that the community was being devastated by malnutrition. Charity workers had plotted the heights and weights of Daasanach children on World Health Organization charts—the same ones our pediatrician used to monitor my daughter’s growth—and determined that more than two-thirds of the kids were malnourished. As a result, families were enrolled in a nutrition program and provided with high-calorie, industrially processed supplements. Yet, as with my daughter, the numbers didn’t align with ordinary observation.
“Everywhere we went, children were running, playing, and laughing,” Pontzer writes. “Kids being kids. They didn’t seem low on energy, nor did they seem particularly short, or ‘stunted.’ ” He saw no other signs of chronic starvation, such as bloated bellies or reduced fertility among adult women. The kids were slim, but in the lanky way typical of so many East African pastoralists.
When Pontzer and his team tracked the growth of Daasanach children, they uncovered patterns that sharply diverged from the W.H.O. curves. At around age two, these kids gain height at rates seldom seen elsewhere in the world. At five, they stand taller, on average, than well-fed kids in Europe and North America. At the same time, they put on weight more slowly, developing lean physiques that are optimal for heat dissipation. Where the German charity diagnosed deficiency, Pontzer saw adaptation.
“Adaptable” offers an engrossing, richly informative exploration of human biological diversity. By revealing how our variable bodies respond to a wide range of environments, it challenges us to rethink universal health benchmarks. These standards inform everything from how we define malnutrition and micronutrient deficiencies to how we estimate the risks of growth abnormalities, metabolic disorders, and cardiovascular dysfunction. They drive global funding priorities, shape international aid programs, and inform social policies. They guide individual clinical assessments, like my daughter’s, and underpin broad statistical claims: seventeen per cent of humans are zinc-deficient; nearly a quarter of Asian-Pacific children are stunted. Yet these benchmarks rest on a monolithic image of human health—a prototypical Homo sapiens whose vulnerabilities remain unchanged across climates and genetic histories. We’ve entered the age of neurodiversity, precision medicine, and “bio-individuality,” but we still assume that malnutrition looks the same in Cologne as it does in rural Kenya. Is it time to move beyond the model of the universal patient?
For decades, pediatricians relied on growth charts for infants and young toddlers which were wildly and obviously flawed. The W.H.O. had endorsed standards developed by the U.S. National Center for Health Statistics based on data from a single American community—Yellow Springs, Ohio. There were questions about their relevance for children elsewhere in the country, let alone the world. But when the W.H.O. released new child-growth standards, in 2006, it appeared that we at last had a truly global benchmark, drawn from studies of children across five continents.
The coördinating team recruited participants from six far-flung locations: Oslo, Norway; Muscat, Oman; Pelotas, Brazil; New Delhi, India; Accra, Ghana; and, as it happens, the city where I live, Davis, California. The researchers maintained strict inclusion criteria—tracking only breast-fed children born to well-off, nonsmoking mothers. The resulting charts gained remarkable traction. By April, 2011, a hundred and twenty-five countries had adopted them, and the United Nations treated them as the new gold standard. Implementation was costly, often requiring countries to overhaul child-health records, retrain medical personnel, and acquire new measurement equipment.
These standards seemed authoritative in part because of their vaunted universality. As the project coördinators wrote in 2006, the standards could be used “to assess children everywhere, regardless of ethnicity, socioeconomic status and type of feeding.” The coördinators also noted a “striking similarity” in the data collected among the six sites, which, given the “built-in ethnic or genetic variability,” affirmed “the standards’ universal applicability.”
Yet how much variability was there, really? The W.H.O. didn’t publish detailed ethnicity information, but, at the time the data were collected, most residents of Oslo, Pelotas, and Davis were of European ancestry. Africa, with more genetic diversity than any other continent, was characterized by a single site. Pacific Islanders, Indigenous Americans, and, most glaringly, East and Southeast Asians were not represented.
The claim of “striking similarity” was also tenuous. The team based its claim on the fact that, at every age, the average height of children at each site was within half a standard deviation of the over-all average. But by that reasoning, as the Indian pediatrician Harshpal Singh Sachdev recently observed in The American Journal of Clinical Nutrition, two sites could differ by as much as a standard deviation and still be considered equivalent. That’s like saying that the mean adult heights in Denmark and Taiwan exhibit “striking similarity” despite differing by more than six centimetres. Among low-income families in urban India, Sachdev noted, ambitious interventions targeting health, sanitation, nutrition, and psychosocial support have failed to increase stature by half a standard deviation, suggesting that differences among sites may reflect disparate physiological baselines.
Beyond height, no cross-site comparisons have ever been published for other measurements, including weight-for-height and weight-for-age metrics and head circumference. Nevertheless, these metrics are regularly used for clinical and cross-national purposes, and treated as if they were universally applicable. When the W.H.O. reports that nearly one in six African children is underweight—or when the Global Nutrition Report states that 45.4 million children under the age of five are wasted—public-health policies are guided by untested assumptions.
My wife and I didn’t know any of this when our daughter was first flagged for being underweight. But we had suspicions that her size might not have been as atypical as the charts implied. My wife’s family, like mine, emigrated from India. Asking around, we learned that many parents of South Asian ancestry had exceptionally small children. On Reddit forums such as r/india and r/ABCDesis, we discovered parents worrying about the same issue. Two of my wife’s cousins had been born smaller than our daughter.
It turned out that credible research corroborated our suspicions. A series of Stanford-led studies had analyzed millions of births in the U.S. and documented a “dual paradox”: U.S.-born women of Mexican parentage, despite having higher risk profiles than U.S.-born women of Indian ancestry, are less likely to have babies with low birth weights. That’s one of many inconsistencies pertaining to size and nutrition. Take the so-called South Asian Enigma: India, Bangladesh, and Nepal exceed most sub-Saharan African countries on key health and development indicators, but their populations still fail to measure up (literally) to those in sub-Saharan Africa or the African diaspora. For instance, Haiti’s infant-mortality rate is almost twice that of India’s, and its per-capita G.D.P. is thirty per cent lower, yet only six per cent of Haitian children are assessed as severely stunted, compared with fourteen per cent of Indian children. You find similar disparities between affluent nations in East Asia and those in northern Europe. Japan and the Netherlands are among the wealthiest countries in the world, with first-rate health care and low disease burdens, but some seven per cent of Japanese children qualify as stunted, compared with only about one per cent in the Netherlands.
The obvious takeaway is that factors aside from living standards—including biological inheritance—are the reason that Dutch and Haitian kids tower over their Japanese and Nepali peers. Yet many researchers have been wary of considering the possibility. In their efforts to resolve the South Asian Enigma, for example, they have busily investigated the effects of open-air defecation, maternal nutrition, and a preference for firstborn sons on the subcontinent. A team of economists examined whether the number of low-weight infants in sub-Saharan Africa who die skews height statistics.
According to Daniel Hruschka, an anthropologist at Arizona State University, none of these theories explain away the discrepancies. Hruschka has long had a personal interest in body measurements. “I consider myself pretty healthy, but if you use B.M.I. guidelines I am obese, and I’ve always wondered, What does that mean for my health?” he told me. The question inspired him to spend more than a decade dissecting anthropometric data, resulting in a slew of revealing findings. In research published in the twenty-tens, he confirmed that a single B.M.I. cutoff for distinguishing normal from obese body weight overestimates obesity, as defined by body fat, in populations with stockier bodies (Pacific Islanders, say) and underestimates it in leaner peoples (South Asians). What’s more, patterns in slenderness, such as similarities between closely related groups and between children and adults in the same group, strongly suggest that genetics plays a major role. In 2016, Hruschka and the anthropologist Craig Hadley, at Emory University, estimated that the standard B.M.I. cutoff misses roughly half a billion overweight people, including some two hundred and fifty million in South Asia alone.
After studying obesity, Hruschka turned his attention to height. In one of his most ambitious projects, published in 2020, he and his former student Joseph Hackman, now at the University of Utah, analyzed measurements from 1.5 million children across seventy countries. Using data on wealth, hygiene, nutrition, and infectious-disease exposure, they calculated each country’s “basal” height-for-age index—the starting height of children living under comparable environmental conditions. If the W.H.O. had been right to assume that children’s potential height is the same everywhere, basal height-for-age measurements should be consistent across populations.
They weren’t. For instance, the basal heights of children in India differed by more than a standard deviation from those of children in Haiti. Even when reared in identical environments, an Indian two-year-old would be expected to be three centimetres shorter than a Haitian two-year-old. When Hruschka and Hackman recalculated rates of severe stunting based on these findings, the estimated prevalence in Haiti more than tripled, from six per cent to twenty per cent. Similarly dramatic increases were observed in West and Central Africa. The reliance on growth charts, it seems, has hidden millions of severe stunting cases in parts of Africa.
These calculations raise another troubling possibility: estimates of stunting in other regions might be exaggerated, leading to ill-advised nutritional interventions. A 2021 study by Sachdev found that more than half of Indian children aged five to nineteen classified as “malnourished” by W.H.O. standards actually show biomarkers of obesity. “Metabolically, they are even overnourished,” Sachdev told me. Where pediatricians would normally recommend cutting back on high-calorie food for such children, “here we are pushing it,” he said.
This blindness to human variation affects children in wealthy countries, too. Though the W.H.O. charts are meant to spot “abnormal growth,” they regularly miss growth disorders in European children. It can’t help that the charts for five- to nineteen-year-olds still draw on decades-old data from the United States. In the Netherlands and Sweden, the W.H.O. charts catch only about seventy per cent of children over the age of five with growth-hormone deficiency; country-specific charts spot around ninety-five per cent. In a 2016 study of nine European nations, the W.H.O. standards consistently failed to outperform local references—except in France, which hadn’t updated its growth charts since 1979.
So charts meant to protect children’s health may be failing them across the globe, missing growth disorders in tall populations while pathologizing normal development in shorter ones. Parents in Mumbai, Manila, and Minneapolis alike must navigate a medical system built on standards that don’t reflect their children’s physiological realities. Some children who need care may be overlooked; others are subjected to unnecessary and potentially harmful interventions.
“Our differences are obvious, even on the surface,” Pontzer observes in “Adaptable.” “Why should our insides be any less diverse?” It’s a reasonable question. We regularly confront the fact that different environments have the power to change us. We know that people who train at high altitudes develop greater aerobic capacity and that populations long exposed to more UV radiation develop darker skin. Pontzer catalogues a great many such examples, from East African hunter-gatherers whose life styles shield them from cardiovascular disease to Southeast Asian sea nomads with genetic adaptations that let them spend hours a day underwater. Yet international organizations continue to operate on the assumption of a universal human physiology—one that, in practice, corresponds strikingly with a Euro-American model.
Take anemia, a condition in which the blood’s ability to carry oxygen is diminished. The W.H.O. first proposed diagnostic cutoffs in a 1959 report. These measures of hemoglobin concentration, a subsequent scientific group admitted, “were chosen arbitrarily,” so new thresholds were introduced after the group reviewed five studies—on American infants, pregnant Canadian women, Norwegian adolescent males, British adults in a mining valley, and, apparently, Swedes. (There’s some uncertainty because the final set of observations was not released.) The revised cutoffs, presented in 1968, were still “somewhat arbitrary,” the authors conceded, but lines had to be drawn. More than fifty years later, these remain the W.H.O.’s guidelines, with only slight modifications for children and pregnant women. A 2023 paper in The Lancet Haematology that announced that nearly two billion people were anemic relied on versions of the 1968 cutoffs.
Untold other benchmarks have similar stories. Criteria for zinc deficiency, as defined by the International Zinc Nutrition Consultative Group, are based on data collected in the United States between 1976 and 1980. Bear that in mind when you hear claims that more than a billion people are zinc-deficient. The threshold for Vitamin D deficiency is also based mostly on research involving Europeans and North Americans, leading to the claim that ninety per cent of Indians lack sufficient Vitamin D, despite the subcontinent’s abundant sunlight.
Why insist on a universal standard? In part, it has been a matter of practicality. For decades, establishing population-specific benchmarks required extensive data collection, statistical modelling, and clinical validation—efforts too costly for most countries to undertake. International organizations like the W.H.O. provided usable, if imperfect, alternatives. But these constraints are disappearing. With vast survey data sets and advanced analytical tools, studies such as those by Hruschka and Hackman reveal population-level patterns that can inform more tailored benchmarks. Meanwhile, scientists in low- and middle-income countries are testing whether inherited global cutoffs line up with local realities. As the barriers to measuring human variation fall, so does the rationale for a one-size-fits-all model.
Even with these advances, Pontzer suspects another reason for the reluctance to discuss biological variation: “Differences are dangerous.” Throughout history, claims of inherent disparities have fuelled oppression, from the justification of slavery to the forced sterilization of the poor. Well-intentioned efforts to account for variation have sometimes harmed marginalized groups. Beginning in 1999, a standard equation for measuring kidney function included a “race coefficient,” which systematically overestimated kidney health in Black patients. As a result, many Black people were referred to specialists belatedly or deemed ineligible for treatments like kidney transplants. In 2021, when the National Kidney Foundation and the American Society of Nephrology recommended removing race from these calculations, more than a million Black Americans were immediately reclassified into more severe stages of kidney disease.
The failures of race-based medicine aren’t an argument for ignoring physiological diversity. Pretending that differences don’t exist doesn’t make them disappear; it only drives practitioners to rely on flawed intuitions. Familiar racial categories do a poor job of tracking ancestry and genetic variation. Yoruba people, in Nigeria, and Bench people, in Ethiopia, both qualify as Black, yet genetically they are further apart than an English person is from a Tamil. Instead of clinging to dubious classifications that obscure variation, we would be better served by developing methods that account for people’s distinctive ancestry and lived environment.
In January, we celebrated our daughter’s first birthday. For reasons of happenstance, we were seeing a new pediatrician for her twelve-month visit. We felt confident. We had ramped up feeding efforts, watching with satisfaction as our daughter’s thighs plumped and her round belly spilled over the waistband of her diaper. Admittedly, every previous visit to the pediatrician’s office had begun with the same sense of achievement—only to be deflated by troubling percentiles. But this time she looked particularly pudgy.
“Seventeen,” my wife whispered, stealing my guess.
“Sixteen pounds, seven ounces,” the nurse read, squinting at the scale. Not bad, I thought.
When the pediatrician entered, he handed us printouts of familiar curves, each marked with a dot representing our daughter: sixth percentile for both weight-for-age and weight-for-height. He asked how these numbers compared with her previous measurements. As we answered him, he stared at the charts, seemingly wrestling with the severity of the situation. Then he said that he was referring us to a dietitian.
Despite ordering a weight check in six weeks and several blood tests, though, he didn’t appear to be visibly troubled. It was as if he, like us, saw two versions of our daughter—the one sitting before him, gleeful and energetic, and the other on the chart, abstractly a cause for concern. Not knowing which to trust, he deferred the verdict to someone else.
Such uncertainty is inherent to medical inference. A heavy baby might just be big-boned; a small one, slim but robust. Yet the reliance on universal benchmarks has widened the disconnect between bodies and their measurements. Resistant to acknowledging population differences, these standards often flag healthy bodies as worrisome and look past malnourished ones. As a result, hundreds of millions of people—often in the poorest countries—are mislabelled, while tools like the W.H.O.’s growth standards, stretched to fit all of humanity, prove less effective than local alternatives. Paradoxically, these efforts sometimes undermine their own goals, concealing, and at times exacerbating, the afflictions of the most vulnerable.
In mid-February, we met virtually with the dietitian. She asked about our feeding routine—which foods, when, how much breast-feeding—and watched as our daughter clambered from my wife’s lap onto the table, reaching for the computer. The dietitian didn’t refer us to another specialist or dwell on percentiles. Instead, she assured us that our daughter was O.K. Petite, yes, but “holding her own.” Besides, she confirmed, many Indian children tend to be smaller. Still, she advised us to keep feeding her well and often, to add butter and the like whenever we could, to stay vigilant.
Maybe in a decade, the one-size-fits-all curves will give way to standards that recognize the different shapes of different populations, and the advice will shift to match. But, for now, we live in the space between two realities—the numbers on a spreadsheet and the child in our arms. ♦
